1. Field of the Invention
The present invention relates to a toner binder for electrophotography to develop electrostatic charge images in electrophotography, electrostatic record, electrostatic printing and the like, and more particularly to a toner for electrophotography that can correspond to a high-speed copier and has high resolution and high image quality, and that is excellent in grindability.
2. Description of the Prior Art
Generally, the method of using electrophotography in a PPC copier or a printer in which a toner image formed on a photo conductor is transcribed on a recording paper (Plain Paper Copy Method) is carried out in such a method that an electrostatic latent image is formed on a photoconductor, then the latent image is developed with toner, and after the toner image is transcribed on a sheet to be fixed like paper and others, the transcribed image is fixed by heating with a heating roll. Because fixing is carried out under heating and pressure, this method can be conducted rapidly and is extremely excellent in heat efficiency. Consequently, the fixing efficiency is very good. However, in this heating roll method, though the heat efficiency is very good, on the other hand, there is such a problem that since toner is contacted with the surface of the heating roll in the melt state of the toner, the toner is transferred by adhering on the surface of the heating roll, and the transferred toner is transferred again on the next sheet to be fixed to contaminate it (an offset phenomenon)
The offset is prevented by, for example, applying silicone oil on the surface of a heating roll with cloth or paper. In this case, the method is very effective in preventing the offset of toner, but because a device is needed to supply a liquid for preventing the offset and the installation of machinery becomes complicated, the repair and management of the machinery becomes complicated to result in an increase in cost, so it is not preferable to adopt such a means. Further, silicone oil and the like may be evaporated by heat to contaminate the inside of the machinery.
Consequently, it is desired to develop toner for a high-speed machine (an oilless fixing method) in a method that is not needed to apply the above-mentioned silicone oil and the like (an oilless fixing method).
On the other hand, a copier is pointed to the direction of high speed, and as a result, the speed of a fixing roll is inevitably high and toner is required to be fixed by heating in a short time. It is necessary for toner to have high fluidity in its melt state in order to be fixed in a short time. Although it is generally effective to lower the glass transition temperature (hereinafer referred to as Tg) of a resin to be used as toner in order to improve the fixing property, on that account there may occur such an undesirable phenomenon as the blocking of toner during storage.
On the other hand, many proposals have been made about toner with the use of a crosslinked polymer as a method for preventing the offset in the development of toner for an oilless fixing method. For example, a method using a crosslinked polymer produced in an emulsion polymerization has been disclosed in Japanese Patent Publication No. 60-36582. In this case, the crosslinked polymer to be used contains 50 to 99 mass % gel part, and when the content of the gel part is increased, the offset resistance is improved but the grindability is worsened, while when the content of the gel part is decreased, the grindability is improved but the offset resistance is worsened. As a result, it was extremely difficult to satisfy both the offset resistance and the grindability.
Moreover, in this method, it is necessary to use a dispersing agent or a dispersing auxiliary agent together to stabilize the emulsified particles when the crosslinked polymer is produced. Since these dispersing agents are highly hygroscopic and adversely affect electric properties, especially charge stability, it is necessary to remove these dispersing agents as much as possible after the crosslinked polymer is produced. However, much labor is required to remove them, and the amount of drainage from the washing facility is also large and needs heavy treatment. Furthermore, U.S. Pat. No. 4,966,829 discloses that it is good to use toner containing a vinyl-based polymer that contains 0.1 to 60 mass % gel component and the molecular weight of the main peak is 1,000 to 25,000 in the soluble part in tetrahydrofuran and that has at least one subpeak or a shoulder in the molecular weight area of 3,000 to 150,000. However, because the method of producing this polymer is a suspension polymerization and also in this case, dispersing agents or dispersing auxiliary agents are used together similarly to an emulsion polymerization, there was the same problem as that in the above-mentioned emulsion polymerization. For this reason, the present inventors have developed a resin by a solution polymerization as toner with a good fixing property (U.S. Pat. No. 4,963,456).
In resin produced by a solution polymerization, the solvent will be removed after the polymerization is ended. Since all of the low volatile components, including unreacted residual monomers and decomposition products of the initiator, can be removed at this time, it is considered that an optimal resin for toner, that is, a homogeneous resin that contains a very low amount of impurities and is electrically stable can be obtained. However, in the production of a crosslinked polymer by a solution polymerization method, there was such a problem that the production became difficult to be performed because of the occurrence of the Weissenberg effect (resins are coiled round a stirring rod). Accordingly, the present inventors have further developed a method of producing a polymer having as high a molecular weight as possible by a bulk polymerization and the like (U.S. Pat. No. 5,084,368). However, there is a limit to the molecular weight of a polymer to be produced, and the offset property had not been conquered completely.
Further, although it is disclosed in Japanese Patent Publication No. 60-38700 that a toner binder produced by heating and mixing a copolymer (A) having 3 to 40% a monomer containing glycidyl group and a crosslinking compound (B) is good, this toner had such a problem in its durability that oppositely charged toner was occurred in a long-term test because of many residuals of epoxy groups.
Furthermore, the present inventors have developed a technique to obtain excellent toner by crosslinking a resin having carboxyl groups produced by a solution polymerization and a compound having a glycidyl group in a specific ratio (Japanese Patent Laid-Open No. 06-011890 and Japanese Patent Laid-Open No. 06-222612). Thus obtained toner can correspond to a high-speed machine, has a good balance of the fixing property, the offset resistance and the blocking property, and is excellent in grindability, production efficiency, electrical properties and charge stability. However, there occurred a problem that because the crosslinking component was subjected to excessive shear to cut gels during kneading in the production step of toner, the elasticity of the toner came to be insufficient at high temperatures, resulting in the worsening of an image after fixing and the failure of obtaining sufficient offset effect.
The present inventors have eagerly studied these requirements to satisfy them, resulting in the development of a technique to obtain an excellent toner binder by improving the molecular weight and epoxy value of a crosslinking agent containing a glycidyl group (Japanese Patent Laid-Open No. 09-319140). Thus obtained toner binder can decrease the cutting of gels during kneading in the production step of the toner, has good effectiveness in the durable developing property and offset resistance, has a greatly improved balance of the fixing property, offset resistance and the blocking property, and is excellent in grindability, production efficiency, electrical properties, and charge stability.
However, at present, marketing needs are pointed toward further high speed and new energy-saving techniques. As a result, it is needed to further lower fixing temperature for the shortening of heating time by high speed technique and to lower even more fixing temperature for energy-saving. That is, since the requirement of the fixing property at further lower temperatures has become strong, it has become difficult to satisfy both requirements of the fixing property at further lower temperatures and the offset resistance needed at the same time by the above-mentioned techniques.
Considering the improvement of the fixing property at lower temperature and the improvement of the balance of the fixing property and the offset resistance with the needs of higher speed and the newly required energy-saving as mentioned above in the copier market as main subjects, it is the object of the present invention to improve all the capabilities of toner for electrophotography, including the fixing property, the offset resistance, the blocking property, the grindability, and the durable developing property.
The present inventors have eagerly studied these requirements to satisfy them and found that in the production of a toner binder for electrophotography that would be obtained by crosslinking a cross linking compound and a copolymer, making the toner binder for electrophotography by stopping the crosslink reaction in the middle of the reaction would improve the fixing property through making the binder lower-viscosity and also improve the offset property by causing the crosslink reaction during of fixing with this remaining crosslink reactivity, and that the grindability, the blocking property and the durable developing property could be improved at the same time. That is, it has been achieved to complete a technique of obtaining a toner binder for electrophotography, which can correspond to a high-speed machine and is excellent in the fixing property, the offset resistance, the blocking property, the grindability and the durable developing property, by making the viscoelasticity curve be concave in the range of 140xc2x0 C. to 180xc2x0 C. and making a minimum value of Gxe2x80x2 0 be present at the bottom of the range, and by specifying the difference between the above described minimum value Gxe2x80x2 0 and storage modulus Gxe2x80x2 200 at 200xc2x0 C.
That is, the present invention can be specified by the matters described in the following.
(1) In a toner binder for electrophotography, wherein when the viscoelasticity of the toner binder is measured in the temperature range of 50 to 200xc2x0 C. and at a heating rate of 2xc2x0 C./min., the viscoelasticity curve in the temperature range of 100 to 200xc2x0 C. showing the relationship between the storage modulus and temperature, in which curve the axis of ordinate is the logarithm (Pa) of storage modulus Gxe2x80x2 and the axis of abscissa is temperature (xc2x0 C.), has a concave in the temperature range of 140xc2x0 C. to 180xc2x0 C. and has a minimum value of storage modulus Gxe2x80x2 at the bottom of the range, this Gxe2x80x2 0 and storage modulus Gxe2x80x2 200 at 200xc2x0 C. have a relationship of Gxe2x80x2 0 less than Gxe2x80x2 200 and the difference xcex94Gxe2x80x2 (Gxe2x80x2 200xe2x88x92Gxe2x80x2 0=xcex94Gxe2x80x2) is 300 Pa or more.
(2) The toner binder for electrophotography described in (1), wherein the above described storage modulus Gxe2x80x2 200 at 200xc2x0 C. is 1000 Pa or more.
(3) The toner binder for electrophotography described in (1) or (2), wherein the toner binder has a glass transition temperature of 45 to 75xc2x0 C., contains 0.1 to 20 mass % gel part, and has a peak in the molecular weight area of 4,000 to 50,000 in the molecular weight distribution based on gel permeation chromatography (GPC) of the soluble part of the toner binder in tetrahydrofuran (THF).
(4) The toner binder for electrophotography described in any one of (1) to (3), wherein the degree of crosslinking reaction is 1 to 50%.
(5) The toner binder for electrophotography described in any one of (1) to (4), wherein the toner binder is obtained by heating and melting a vinyl resin (A) containing glycidyl groups, the weight-average molecular weight of which resin is 10,000 to 100,000 and the epoxy value of which resin is 0.005 to 0.1Eq/100g, and a vinyl resin (B) containing carboxyl groups, the acid value of which resin is 1 to 30 mg KOH/g and the glass transition temperature of which resin is 40 to 70xc2x0 C., to be crosslinked by the use of the above described vinyl resin (A) containing glycidyl groups as a crosslinking agent.
(6) The toner binder for electro photography described in any one of (1) to (5), wherein one of styrene-acrylic resins is a main component.
(7) Toner for electrophotography, wherein the toner binder for electrophotography described in any one of (1) to (6) is used.